This invention relates to erosion control systems, and more particularly to an erosion control system which utilizes a plurality of hexagonal blocks having tongue and cavity coupling means to form an entire revetment comprised of hand-placed blocks and/or preassembled machine-placed interlocking hexangular block mats.
The erosion of natural and artificial channels, beaches, and other points where water interfaces with soil is a frequently encountered and much studied problem. Erosion can be the result of abrasion, which is the removal of material from the surface of a bank. The primary cause of abrasion is the movement of water along the soil/water interface, with contributing factors being high velocities, currents, waves, long-eddies and boat wash.
Various revetment systems have been used in attempts at preventing, or at least slowing, erosion. Randomly sized concrete chunks, or "riprap", have been placed along riverbanks and beaches in attempts to slow erosion. Too often, though, the chunks would be too large and some erosion would still occur. Similarly, attempts at paving have been futile due to the destructive effects of hydrostatic pore pressure.
Recently, revetment constructions utilizing interconnected blocks have become known. These constructions typically involve placing blocks of various shapes into a mat which in turn, is placed along the riverbank or beach. These mats make intimate contact with the underlying soil during settlement and prevent realignment of the slope by wave and current action. However, because such constructions have ignored one or more basic considerations, there has yet to exist a truly effective means of preventing hydrodynamic failures due to waves and currents.
One overlooked consideration involves the "uplifting" of entire revetments due to hydrostatic pore pressure. When water passes between the bottom of a revetment, or an individual block, and the earth, hydraulic action takes place. This, for example, results when waves of passing vessels and natural variable frequency and wave heights cause turbulence, thereby affecting water pressures under the revetment and in the subsoil. When the uplift pressure forces become greater than the sum of the weight of the block and its friction forces, a loss of stability occurs, and one or more blocks can be lifted from the revetment.
A second overlooked consideration is that the interconnected blocks must be permitted to shift within reasonable bounds within the mat so as to avoid any individual block taking the entire destructive force outlined above, and yet be restrained so as not to become dislodged. If firmly restricted, the interconnecting members of the block are apt to break off or sheer when the blocks move during hydraulic action, which in turn can result in the dislocation of the block and the eventual loss of an entire revetment. This is especially important when concrete, which is low in tensile strength, is used to produce the blocks.
Another overlooked consideration relates to the means used to interlock the blocks. Reinforcing or connecting rods and cables made of material subject to corrosion, such as steel, are traditionally used because unlike plastic, such materials best withstand attempted vandalism and do not break down upon exposure to sunlight. However, corrosion of such cables, when surrounded in concrete, causes the concrete to expand, which in turn results in spalling. Once spalling of the concrete takes place, the blocks are apt to crack or disintegrate and the entire revetment can be lost. Attempts at replacing such cables using blocks having interconnecting members have been made, but all have failed. Such interconnections have involved either solely horizontal locking members or have failed to allow the movement of members outlined above, or both.
Another important, yet unmet, consideration is cost effectiveness. Any efficient erosion control system must have low production and application costs. To keep costs low, the blocks must be of such design that they can be quickly assembled into a mat at a desired location in a systematic fashion without auxiliary components and by relatively unskilled labor.
There exists a need, therefore, for a block-formed revetment mat which is sufficiently stable so that no part can be displaced, sufficiently flexible so that the mat can bend to a limited extent without losing mutual connection between the blocks, sufficiently durable so as not to break apart or disintegrate, and economical in that it can be manufactured and applied quickly and at low cost.